Sulphuric acid plants, and other applications involving the drying of chlorine or sulphur dioxide, commonly include packed towers in which concentrated sulphuric acid is circulated over a body of packing. Such towers have been used for more than one hundred years, and many different approaches to the construction of such towers have been attempted, with varying degrees of success.
The basic criteria that a good distribution system must meet are (1) the need to spread the liquid uniformly over the packing with a minimum reliance on the packing as a liquid spreading medium, (2) the need to maintain good distribution regardless of acid flow, (3) the ability of the device to demonstrate good distribution without complex tests, (4) the ability of the device to be cleaned of any accumulations of sulphate, brick chips, packing chips or other rubbish without a major maintenance effort, and (5) resistance to corrosion and erosion such that good distribution is present not only when the device is new but also when it has eroded sufficiently to be near the end of its life span. The device should also be simple, easy to repair or replace, durable, and made of reasonably available economical materials.
An early method used to distribute acid was a cast iron pipe distributor. This device used a central header which fed arms which in turn had drilled holes which allowed the acid to flow out on to the packing. The header and arms were made from common cast iron fittings such as tees, crosses and pipe. Acid velocities from the holes were in the range of three to six feet per second. While the device was relatively economical compared with other alternatives, it blocked access to the packing since the pipes and header covered approximately 50 per cent of the tower cross-section. The arrangement also required significant mechanical support.
Other defects in the cast iron system included the lack of erosion resistance in the cast iron which resulted in holes sizes increasing to the point of affecting acid distribution. There was also some tendency for the collection of tramp ceramic, brick and sulphate material in the arms where it could not be seen and from which it was difficult to remove.
The cast iron system was later modified by the insertion of corrosion resistant materials in the holes as discrete inserts. Subsequently cast iron was displaced completely by the use of very corrosion resistant materials such as high silicone containing austenitic steels. These materials needed no inserts since they are very corrosion and erosion resistant in strong sulphuric acid. However these designs are still difficult to clean, and they block a fraction of the tower cross-section which can typically be as high as 45 per cent.
A relatively common approach in North American practice has been to use troughs from which the acid overflows through downcomer spouts onto the packing. Since the spouts are open at the top, it is relatively easy to see any build-up of tramp materials and to remove such materials. However the acid distribution is then severely dependant on the leveling of the individual troughs, the absence of any sagging, and the absence of any entrance effects. Acid distribution is also needed outside the tower to ensure that each individual header receives an appropriate share of the total acid flow. The troughs are usually not corrosion resistant and replacement is necessary from time to time. Moreover the troughs are normally specially designed for each tower so that repair or replacement is very difficult. The design can be improved, but at significant cost, by using more corrosion resistant materials. A newer version is also available in which the acid flows outwardly through submerged holes, thus reducing the sensitivity of the device to levelling.